This invention relates to power supplies, and more particularly to power supplies for high-voltage pulse applications, such as in a radar system using a travelling-wave tube transmitter.
Radar systems have been used for more than fifty years, and during that time many varieties have emerged, including continuous-wave varieties. Many radar systems continue to use relatively short, high-power pulses of radio-frequency electromagnetic radiation to detect, locate and track targets. Modern radars provide very sophisticated features and capabilities at long range on small targets. In order to provide such features and capabilities, the recurrently transmitted electromagnetic pulses are often required to meet stringent frequency, power, and stability criteria while executing a recurrent program involving changing frequency and power.
In general, high performance in a radar system depends upon having a large bandwidth, so that at some times very short-duration pulses can be transmitted for making fine determinations of distance and dimensions, and at other times much longer-duration pulses can be transmitted for long-range detection. At one time, radar systems used simple vacuum tubes in their transmitters, but the power and frequency limitations of such tubes made them somewhat unsatisfactory. The development of the klystron and magnetron provided increased power at high frequencies, but with limited bandwidth. Modern radar systems use broadband sources of transmitted electromagnetic radiation, which are often in the form of one or more travelling-wave tubes (TWTs), and sometimes of arrays of solid-state transistors. At the current state of technology, the highest power with wide bandwidth is available with travelling-wave tubes.
FIG. 1 is a simplified diagram in block and schematic form, illustrating a prior-art travelling-wave tube power supply 10. In general, stability of the transmitted pulse from a travelling-wave tube, such as 12 of FIG. 1, can be achieved by maintaining constant the various TWT electrode voltages. In the simplest type of high-power TWT transmitter, a travelling wave tube, such as 12, is energized by a voltage supply, such as 14 of FIG. 1, connected to its cathode 12ca and collector 12co. Such an arrangement sets the power dissipation of the TWT to equal the product of the tube current and the voltage of source 14. The actual tube voltage and power are often determined by considerations including the best linearity or least distortion introduced by the tube. In an actual system, the voltage of power supply 14 may be about 35 kilovolts (kv). This type of supply can be effective, but the unavoidable internal impedance of the power supply 14 undesirably results in a decrease of the applied voltage each time an energy pulse is produced by the TWT 12.
In order to reduce the voltage drop attributable to the internal impedance of the power supply 14 of FIG. 1, the prior art parallels the power supply with an energy storage capacitor, illustrated as 16 in FIG. 1. The energy storage capacitor tends to reduce the instantaneous voltage drop at the inception of the pulse, but a voltage decrease or xe2x80x9cdroopxe2x80x9d still occurs over the duration of the transmitted pulse. A sufficiently large energy storage capacitor 16 can, however, keep the voltage droop within acceptable limits. The presence of the energy storage capacitor 14, in turn, results in the possibility of a large current discharge in the event of a short-circuit or flashover within the TWT. A current-limiting resistor 18 is introduced in a serial connection between the TWT cathode and the combination of the power supply 14 and the capacitor 16, to prevent damage to the TWT and power supply.
It should be noted at this point that the terms xe2x80x9cbetweenxe2x80x9d and xe2x80x9cacross,xe2x80x9d and certain other terms, have meanings in electrical usage which are different from those commonly used. More particularly, the terms have meanings which are not related to physical placement, but rather relate to the terminals to which electrical coupling is made. Thus, signal flow xe2x80x9cbetweenxe2x80x9d A and B takes place if the signal leaves one of A and B and arrives at the other, regardless of whether the path taken happens to lie on, or pass through, a straight line extending from A to B. Those skilled in the art know this so thoroughly that little though is given to the use of the terms, and they are automatically understood.
It has been found that greatest modulation sensitivity (somewhat corresponding to xe2x80x9cgainxe2x80x9d) of a travelling-wave tube occurs at specific values of voltage between the cathode 12ca and the body 12b of the tube 12. The body 12b of the TWT 12 should be grounded, for reasons of safety and to reduce the possibility of flashover. In general, the maximum-modulation-sensitivity cathode-to-body voltage does not correspond with the optimum cathode-to-collector voltage of the TWT. In order to obtain maximum modulation sensitivity of TWT 12 of FIG. 1, a second voltage or power supply 20 is provided, with its negative (xe2x88x92) terminal coupled to the collector 12co of TWT 12 by way of a current-limiting resistor 22, and with its positive (+) terminal coupled essentially to ground, by way of a control or regulator arrangement designated generally as 30. The voltage produced by power supply 20 introduces an offset voltage which drives all the voltages associated with the TWT in a negative direction, except for the ground voltage at 12g applied to the TWT body connection 12b. The voltage of power supply 20 is selected to a value which sets the cathode-to-ground voltage to about the maximum-modulation-sensitivity voltage, which is to say that the floating voltages of power supply 14 are driven negative until the cathode 12ca of TWT 12 is at the desired negative voltage relative to the ground voltage at body terminal 12b. In one application, the voltage of power supply 20 is about 8 kilovolts, which sets the cathode-to-ground voltage at about xe2x88x9243 kv. The voltage offset introduced by power supply 20 does not change the power dissipation of the TWT, because power supply 14 continues to establish the voltage (about 35 kv in the example) between cathode 12ca and collector 12co. Thus, the arrangement of FIG. 1 as so far described maintains the cathode-to-body voltage of the TWT near the optimum for modulation sensitivity, and the cathode-to-collector voltage at some value optimized for a combination of signal distortion and TWT dissipation.
In the arrangement of FIG. 1, power supply 20 is also associated with a collector energy storage capacitor 24, which serves the same purpose for power supply 20 as energy storage capacitor 16 serves for power supply 14. It will be appreciated that the internal impedances of power supplies 14 and 20 necessarily result in reduction of TWT voltage when current is drawn, whether or not the power supplies are paralleled by energy storage capacitors. For very short pulses, such energy storage capacitors can be very helpful in ameliorating voltage drop. However, as the desired transmitted pulses become longer (of longer duration or of increased duty cycle), the size of the requisite energy storage capacitance can itself become a problem. In a large, high-power radar system using an energy storage capacitor across the cathode-to-collector path of a TWT, the weight of energy storage capacitors can be hundreds of pounds, which may be undesirable for some applications.
To the extent that transmitted frequency and power do not conform to the desired pattern or program because of variation in the power supply voltages of the TWT of a radar transmitter, signal processing can in some instances be used to compensate for the resulting deficiencies. In general, reduction in the amount of signal processing is desirable, both for reducing the amount of processing power required, and therefore the costs of the system, and for increasing the processing speed, thereby allowing improved performance. The need for additional processing in order to compensate for deficiencies in the radar transmitter characteristics, then, suggests that it is desirable to further stabilize the power supplies associated with a TWT transmitter. As mentioned, one way to do this is to increase the sizes of the energy storage capacitors, but this may not be desirable or possible, and the desired level of stability of the cathode 12ca voltage relative to ground at the body terminal 12b may not be achievable without very large energy storage capacitors. The magnitude of the problem may be understood by considering that the allowable variation of the cathode-to-ground voltage may be one or two volts out of 40 or more thousands of volts. An additional aspect of the problem associated with the use of ever-larger energy storage capacitors is that the arrays of capacitors, by virtue of their large size, introduce irreducible inductance into the power-supply circuits, which adversely affects their effectiveness in reducing droop in the presence of very short-duration pulses.
In the power supply of FIG. 1, the control circuit 30 provides a measure of feedback voltage control which tends to reduce the effective or dynamic internal impedance of the combined power supplies 14 and 20. In general, control circuit 30 includes a controllable resistance element 31 consisting of a control FET 32 having its source connected to ground by way of a sensing resistor 33 and its drain connected to the positive (+) terminal of power supply 20 by way of a tetrode 34. The gate 32g of FET 32 is connected to the output port 36o of a cathode voltage control circuit illustrated as a box 36. A cathode voltage sensing input port 30c of block 36 is connected, by a sense conductor or xe2x80x9clinexe2x80x9d 36s, to sense the xe2x80x9ccathodexe2x80x9d voltage at the negative (xe2x88x92) terminal of power supply 14. Block 36 internally compares the cathode voltage with a reference voltage to produce an error voltage at its output port 36o, and provides the error voltage to the gate 32g of FET 32, all in known fashion. Tetrode 34 is connected between the drain of FET 32 and the positive terminal of power supply 20 to keep the FET drain voltage within a range which the solid-state FET 32 can handle. When the beam voltage regulator 30 of FIG. 1 is in operation, the dynamic impedance of the combined power supplies 14, 20 supplying the cathode 12ca of the TWT 12 relative to ground is much reduced from that which it would have in an open-loop or unregulated condition. The reduction in effective internal impedance of the combined power supplies may be used directly to improve the voltage stability at the cathode 12ca of TWT 12, or may be used to reduce the sizes of the energy storage capacitors, or both.
It should be noted that each of the power supply or voltage source blocks 14 and 20 of FIG. 1 may have their own internal negative-feedback controls for setting their output voltages and for reducing their effective internal impedances.
As performance requirements of radar systems increase, with increasing requirements for both short- and long-pulse operation, it has been found that the power supply 10 of FIG. 1 can no longer be used, because the shorter transmitter pulses are outside the useful bandwidth of the controller. In some cases, operation with short pulses may even cause regeneration, which may require that the feedback control circuit be deactivated during the transmitted pulse.
A power supply for a travelling-wave tube (TWT) according to an aspect of the invention provides voltage for the cathode-to-collector beam of a travelling-wave tube. The travelling-wave tube includes a cathode, a collector, and a body connected to ground or other reference voltage source. The power supply includes a first voltage source including a negative (xe2x88x92) terminal and a positive (+) terminals coupled to the collector of the travelling-wave tube. An electrical coupling arrangement, such as a conductor or a current-limiting resistor, is coupled to the negative terminal of the first voltage source and to the cathode of the travelling-wave tube, for thereby establishing a cathode-to-collector voltage of the travelling-wave tube at a value near the first voltage. The power supply also includes a controllable impedance including a first terminal coupled to the ground (or other reference) and also including a second terminal. The controllable impedance further includes a control terminal responsive to a control signal for controlling the impedance between the first and second terminals of the controllable impedance. In a particular embodiment, the controllable impedance comprises (a) a sensing resistor having one end coupled to the ground (or other reference); (b) a first controllable solid-state device including first and second electrodes, and a control electrode to which a control signal can be applied for controlling the impedance between the first and second electrodes. The first controllable solid-state device has its first electrode coupled to the ground (or other) reference by way of the sensing resistor, and also has its control electrode coupled to (or in common with) the control terminal of the controllable impedance. The controllable impedance also includes (b) at least two additional controllable solid-state devices, each of the additional controllable solid state device including first and second electrodes and a control electrode to which a control signal can be applied for controlling the impedance between the first and second electrodes. The additional controllable solid-state devices are in a cascade in which each such cascaded additional solid-state device, except those at the ends of the cascade, has its second electrode coupled to the first electrode of the next adjacent one of the additional controllable solid-state devices in the cascade, and in which the first electrode of that one of the additional controllable solid-state devices at a first end of the cascade is connected to the second electrode of the first controllable solid-state device, and in which the second electrode of that one of the additional controllable solid-state devices at a second end of the cascade is connected to, or common with, the second terminal of the controllable impedance. Additionally, the controllable impedance includes (c) means for equalizing the voltages applied between the first and second terminals of the additional controllable solid-state devices. The power supply further includes a second voltage source including a negative (xe2x88x92) terminal coupled to the collector of the travelling-wave tube and a positive (+) terminal connected to the second terminal of the controllable impedance, and a cathode-to-ground voltage controller coupled to the cathode of the travelling-wave tube, to the ground, and to the control terminal of the controllable impedance, for controlling the control signal in a manner which tends to maintain the voltage between the ground and the cathode of the travelling-wave tube constant. In an embodiment of the invention, the voltage controller is of the feedback type.
In a preferred embodiment, the power supply further comprises a capacitance arrangement coupled across the first voltage source. In another embodiment, the means for equalizing the voltages includes a resistive voltage divider defining plural taps, with the taps of the resistive voltage divider being connected to the control electrodes of the additional controllable solid-state devices. In yet another embodiment of the invention, a second controllable impedance includes a first terminal coupled to the ground (or other reference) and also includes a second terminal coupled to the negative terminal of the second voltage source, the second controllable impedance further including a control terminal responsive to a control signal for controlling the impedance between the first and second terminals of the controllable impedance, and means are provided for coupling the cathode-to-ground voltage controller to the control electrodes of the first-mentioned and second controllable impedances, for parallel control of the first-mentioned and second controllable impedances. In one manifestation of this last embodiment, means are provided for tending to equalize the current through each of the first-mentioned and second controllable impedances, and in one version, this means includes a first sensing resistor coupled between the ground and the first electrode of the first controllable solid-state device for developing a signal representing the current through the first controllable solid-state device, and a second sensing resistor connected between the first terminal of the second controllable impedance and the ground for developing a signal representing the current through the second controllable impedance, together with first and second amplifiers, each including an inverting input port and a noninverting input port, the inverting input ports of the first and second amplifiers being connected to the first and second sensing resistors, respectively, and the noninverting input ports of the first and second amplifiers being connected in common to the cathode-to-ground voltage controller for receiving the control signal therefrom.
Another avatar of the invention lies in a power supply for the cathode-to-collector beam of a travelling-wave tube including a cathode, a body connected to ground (or other reference potential), and a collector. In this avatar, the power supply comprises a first direct voltage source including a negative terminal, and also includes a positive terminal coupled to the collector of the travelling-wave tube. An electrical coupling means is coupled to the negative terminal of the first voltage source and to the cathode of the travelling-wave tube, for thereby establishing a cathode-to-collector voltage of the travelling-wave tube at a value near, or ideally at, the first voltage. A first controllable impedance includes a first terminal coupled to the ground (or other reference) and also includes a second terminal. The first controllable impedance further includes a control terminal responsive to a control signal for controlling the impedance between the first and second terminals of the first controllable impedance. A second controllable impedance includes a first terminal coupled to the ground (or other reference) and also includes a second terminal. The second controllable impedance further includes a control terminal responsive to a control signal for controlling the impedance between the first and second terminals of the second controllable impedance. A second voltage source includes a negative terminal coupled to the collector of the travelling-wave tube and a positive terminal connected to the second terminal of the first and second controllable impedances, and a cathode-to-ground voltage controller coupled to the cathode of the travelling-wave tube, to the ground (or other reference), and to the control terminals of the first and second controllable impedances, for controlling the control signal in a manner which tends to maintain constant the voltage between the ground (or other reference) and the cathode of the travelling-wave tube.
In a particular manifestation of this avatar, a capacitance means is coupled across the first voltage source. In another particular manifestation, the coupling means comprises a resistance. In one version of this avatar, each of the controllable impedances comprises (a) a resistor having one end coupled to the ground or other reference, (b) a first controllable solid-state device including first and second electrodes, and a control electrode to which a control signal can be applied for controlling the impedance between the first and second electrodes. This first controllable solid-state device has the first electrode coupled to the ground (or other reference) by way of the resistor, and also has the control electrode coupled to the control terminal of the controllable impedance. This version of the avatar also includes (b) at least two additional controllable solid-state devices, where each the additional controllable solid state device includes first and second electrodes and a control electrode to which a control signal can be applied for controlling the impedance between the first and second electrodes. The additional controllable solid-state devices are coupled in a cascade in which each such cascaded solid-state device, except those at the ends of the cascade, has its second electrode coupled to the first electrode of the next adjacent one of the additional controllable solid-state devices in the cascade, and in which the first electrode of that one of the additional controllable solid-state devices at a first end of the cascade is connected to the second electrode of the first controllable solid-state device, and in which the second electrode of that one of the additional controllable solid-state devices at a second end of the cascade is connected to the second terminal of the controllable impedance; and (c) means for equalizing the voltages applied between the first and second terminals of the additional controllable solid-state devices.